The "fred tool" part of the rail runner is for the bottom tuck only. The bottom tuck is normally anywhere from a 3/8" - 3/4" radius (depending on your preference) and the rail bands are cut in the the rail runner.
The deck rail bands are much bigger and not a "radius" but an arc or curve. The rail bands are created with a power planer or G-rasp tool and the sharp corners of the rail bands are cut down by rail screen (you should be using a more aggressive screen like the sand shark screen to shape the rails down... they'll come out round and less angular or boxy)
Think of the rail bands as facets of a diamond. Flat surfaces intersecting at points. There is a curve hiding in there that is exposed when the corners are screened off.
More info and dimensions for rail band shaping in our Surfboard Building Guide